This invention relates to color printing with a computer printer, color copier or the like. More particularly, this invention relates to a method of providing full resolution, multiple color printing with less ink and at a faster rate than is conventionally required.
Printing resolution is defined by the number of dots printed per inch (dpi), with 300 dpi being typical full resolution. Printing at full resolution is usually done with a dot on dot (DOD) technique, where colors are generated by printing one or more color dots at each grid point (dot location) on the printing medium such as paper. For primary colors, only one dot is needed for a grid point, but for secondary colors, which are combinations of primary colors, at least two dots of different colored inks are required at a grid point.
The DOD technique has a number of drawbacks. It requires twice as much as ink as desirable to printed at each secondary color dot at a grid point. This results in excessive ink use that can lead to unwanted ink mixing at adjacent grid points and to paper wrinkling. It is also relatively slow, since secondary colors take twice as long to print as primary colors. One modification that can reduce ink use and paper wrinkling is to print each grid point with two dots of half strength. Both primary colors and secondary colors are then printed with two dots. But this approach further slows the rate of printing since every grid point to be printed requires two passes of the printhead.
An alternative to the DOD technique is the dot next to dot (DND) technique. In its conventional form, DND provides the same number of colors as DOD, but not at resolution of DOD. Primary colors are printed as in DOD; secondary colors are printed by placing the requisite primary colors adjacent to each other in a checkerboard or other pattern to give the appearance of secondary color. For example, green, a secondary color, may be printed by forming a pattern of alternating color dots of cyan and yellow on the pattern. The DND technique thus avoids the need to print two colors on a single grid point to create secondary colors. However, the cost is resolution. Images formed of secondary colors with DND are printed with only half the spatial resolution of DOD.
Other printing attempts to print secondary colors include various dithering or error diffusion techniques that require a great deal of software processing, making them relatively slow and complex.
The drawbacks associated with these attempts and with conventional DND are unacceptable for a low cost, high quality color printing device. The present invention, on the other hand, uses a novel printing technique that provides the apparent resolution of DOD without the drawbacks described above.